40 



GUIDEBOOK OF THE WESTERN UNITED STATES. 



to the sea. 



1 



Igneous rocks appear near by on the right m the di\^ide 



Ward 



The railroad now enters the Umpqua basm and conthiues in it for 

 over 100 miles. In the descent do^\Ti Pass Creek may be seen out- 

 crops 



of tujffaeeous conglomerate 



(formed of volcanic fragments), 

 which is quarried here and there for riprapping the roadbed. As 

 the gcncTixl dip of these rocks is northeastward, successively older 

 strata appear in the course of the southwestward descent. Near 

 milepost G19, on the rifrht, is an old quarry in sandstone which con- 



numerous 



At Comstock there are traces of coal in the sandstones, and a few 

 Eocene shells have been found. The valley of Pass Creek in <^eneral 



r 



Comstock. 



Elevation 476 feet. 

 Population 321 * 

 Seattle 339 miles. 



IS narrow 



timber, H 



fine 



and 



and therO; however, it widens, and 



the hillsides . Lumh 



of the region. 



between Comstock and Drain. 



Anlauf, Safely, and Leon a are passed 



through the Coast Range by the Umpqua 

 shows certain features that throw light on 

 the later geologic history of the region. 

 In the first place, if the section given in 

 figure 7 "were extended it would show that 

 the summits of the hills ncrth and south of 

 the river are more or less flat and at nearly 

 the same level. An observer standing on 

 one of these hills and lookinsr over the 



^ A cross section of the valley cut land apparently ceased for a time and the 



'^ ' - -' ^£^^^^ stopped cutting and began to mean- 



der and thus to widen its valley. Later 

 the upward movement of the land was 

 renewed and the river again began to cut 

 down its bed, leading remnants of its old 

 valley floor as benches or terraces along 

 the sides of its present valley. 



A similar relation of peneplain, terrace, 

 and present valley bottom can be deci- 

 phered along nearly all the larger streams 

 of the Klamath MountaLis and adjacent 

 parts of the Coast Range. This shows 

 that a large area was affected by the same 

 vertical movements of the land. 



The Umpqua flows through the Coast 

 Range to the sea in a direct course, which 

 presents a strong contrast with its mean- 

 dering course east of the range. This dif- 

 ference is due to the fact that in the rela- 

 tively hard rocks of the Coast Range the 

 stream has been busily cutting down and 

 has not remained long enough at any one 

 level to widen its valley greatly. In the 

 east of the Coast Range th^ river has 



Figure 7.— Generalized section of Umpqua River 

 vaUey in Coast Range, Oreg. a. Peneplain; 6, 

 eariier valley; c, later valley. 



country could disregard the present val- 

 leys and imagine that his vision was 

 sweeping over a vast rolling surface that 

 was nearly level — a peneplain. The im- 

 agination would merely be recalling what 

 was once reality — a land surface worn 

 down nearly to sea level through long 

 erosion by rain and streanos. The Umpqua 

 flowed across this surface to the'sea. 



The next event was a slow rise of the 

 land, in consequence of which the river 

 began to flow faster and to deepen its val- 

 ley. After it had cut down to a consider- 

 able depth the upward movement of the 



probably flowed over softer rocks and at 



stages in its history has had time, 



downward 



swing 



valley, and establish graceful meandera 

 after the common habit of low-grade 

 streams. 



